Multifunctional anodized layer

ABSTRACT

A method of anodizing includes immersing an aluminum alloy workpiece in a phosphoric acid anodizing solution and applying a voltage to form a porous oxide layer on the workpiece. The workpiece is then removed from the phosphoric acid anodizing solution and immersed in a controlled anodizing solution. A voltage is applied to form a dense oxide layer under the porous oxide layer. Dissolution of the porous oxide layer is controlled during the formation of the dense oxide layer by using tartaric acid in the controlled acid solution such that the thickness of the porous oxide layer is substantially equivalent before and after the formation of the dense oxide layer. The duplex anodized layer can be further sealed by soaking in a sealing solution to protect the porous oxide layer from hydrolytic decomposition, to improve corrosion protection, and to enhance the bonding with other structural components through adhesives.

CROSS-REFERENCE TO RELATED APPLICATION

The present disclosure claims priority to U.S. Provisional PatentApplication No. 62/026,823, filed Jul. 21, 2014.

BACKGROUND

This disclosure relates to anodizing aluminum alloys.

Anodized coatings are used to protect aluminum alloys from corrosion andto provide good adhesive bond strength. Some anodized coatings providerelatively good corrosion protection, but also have a relatively smoothsurface that does not promote good bonding strength. Alternatively,other anodized coatings are textured and thus have good bonding strengthbut are porous and do not provide good corrosion resistance.

SUMMARY

A method of anodizing according to an example of the present disclosureincludes immersing an aluminum alloy workpiece in a phosphoric acidanodizing solution, and applying a voltage to the aluminum alloyworkpiece in the phosphoric acid anodizing solution. The phosphoric acidanodizing solution and the voltage act to form a porous oxide layer onthe aluminum alloy workpiece. The method includes the steps of removingthe aluminum alloy workpiece from the phosphoric acid anodizing solutionand then immersing the aluminum alloy workpiece in a controlledanodizing solution, and applying a voltage to the aluminum alloyworkpiece in the controlled anodizing solution. The controlled anodizingsolution and the voltage act to form a dense oxide layer on the aluminumalloy workpiece under the porous oxide layer. The method includes thestep of controlling dissolution of the porous oxide layer during theformation of the dense oxide layer by using tartaric acid in thecontrolled acid solution such that the thickness of the porous oxidelayer is substantially equivalent before and after the formation of thedense oxide layer.

In a further embodiment of any of the foregoing embodiments, thecontrolled anodizing solution includes the tartaric acid and sulfuricacid.

In a further embodiment of any of the foregoing embodiments, the step ofapplying the voltage to the aluminum alloy workpiece in the controlledanodizing solution includes ramping the voltage to a predetermined holdvoltage within three minutes, and then holding at the predetermined holdvoltage for no more than 30 minutes.

In a further embodiment of any of the foregoing embodiments, thecontrolled anodizing solution has a temperature of 20-35° C. during thestep of applying the voltage.

In a further embodiment of any of the foregoing embodiments, thetartaric acid has a concentration in the controlled acid solution of60-100 gram/L.

In a further embodiment of any of the foregoing embodiments, thecontrolled anodizing solution consists essentially of the tartaric acidand sulfuric acid.

In a further embodiment of any of the foregoing embodiments, thecontrolled anodizing solution has a ratio of the tartaric acid to thesulfuric acid from 1:1 to 4:1.

In a further embodiment of any of the foregoing embodiments, thecontrolled anodizing solution has a ratio of the tartaric acid to thesulfuric acid of approximately 2:1.

In a further embodiment of any of the foregoing embodiments, thephosphoric acid anodizing solution is a 7.5 volume % phosphoric acidaqueous solution, and the phosphoric acid anodizing solution is at roomtemperature of 20-25° C. during the step of applying the voltage to thealuminum alloy workpiece in the phosphoric acid anodizing solution.

In a further embodiment of any of the foregoing embodiments, thephosphoric acid anodizing solution consists essentially of an aqueousphosphoric acid solution, and the controlled anodizing solution consistsessentially of the tartaric acid and sulfuric acid.

A further embodiment of any of the foregoing embodiments includesimmersing the aluminum alloy workpiece that has the porous oxide layerand the dense oxide layer in a nitrilotrismethylene solution.

A further embodiment of any of the foregoing embodiments includesimmersing the aluminum alloy workpiece that has the porous oxide layerand the dense oxide layer in an aqueous trivalent chromium-containingsealing solution to deposit a chromium compound in the dense oxidelayer.

An anodized article according to an example of the present disclosureincludes an aluminum alloy substrate with a surface portion that isconverted to a porous oxide layer of aluminum oxides/phosphates, a denseoxide layer under the surface portion, wherein the porous oxide layer ofaluminum oxides/phosphates and the dense oxide layer together are aduplex coating that has an electric resistance of at least 10⁹ Ohms, andan electrically conductive material adjacent the duplex coating. Theelectrically conductive material is different in composition from thealuminum alloy, and the electric resistance of the duplex coatingprovides a galvanic corrosion barrier between the aluminum alloysubstrate and the electrically conductive material.

In a further embodiment of any of the foregoing embodiments, the denseoxide layer includes residual tartaric acid and sulfate ions.

In a further embodiment of any of the foregoing embodiments, the denseoxide layer is sealed with a chromium compound.

In a further embodiment of any of the foregoing embodiments, the denseoxide layer is thicker than the porous oxide layer.

An anodized airfoil according to an example of the present disclosureincludes an aluminum alloy airfoil extending between a leading end and atrailing end, with at least a surface portion of the leading end beingconverted to a porous oxide layer of aluminum oxides/phosphates, a denseoxide layer under the surface portion, wherein the porous oxide layer ofaluminum oxides/phosphates and the dense oxide layer together are aduplex coating that has an electric resistance of at least 10⁹ Ohms, anda sheath formed of an electrically conductive material and mountedadjacent the duplex coating at the leading end of the aluminum alloyairfoil. The electrically conductive material is different incomposition from the aluminum alloy, and the electric resistance of theduplex coating provides a galvanic corrosion barrier between thealuminum alloy airfoil and the electrically conductive material of thesheath.

In a further embodiment of any of the foregoing embodiments, the denseoxide layer includes residual tartaric acid and sulfate ions.

In a further embodiment of any of the foregoing embodiments, the denseoxide layer is sealed with a chromium compound.

In a further embodiment of any of the foregoing embodiments, the denseoxide layer is thicker than the porous oxide layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the present disclosure willbecome apparent to those skilled in the art from the following detaileddescription. The drawings that accompany the detailed description can bebriefly described as follows.

FIG. 1 illustrates an example method of anodizing.

FIG. 2 illustrates an example anodized article.

FIG. 3 illustrates another example anodized article.

FIG. 4 illustrates a micrograph of an in-process workpiece during themethod of FIG. 1.

FIG. 5 illustrates a workpiece after the method of FIG. 1.

FIG. 6 illustrates an anodized airfoil.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates an example method 20 of anodizing analuminum alloy workpiece. As will be described, the method 20 can beemployed to anodize the aluminum alloy workpiece to provide goodcorrosion resistance, good bonding strength, and good electrical barrierproperties.

As will be appreciated, the steps or actions described with respect tothe method 20 can be employed with additional steps or other processesas desired. In this example, the method 20 includes a first immersionstep 22, a first voltage application step 24, a second immersion step26, a second voltage application step 28, a third immersion step 30, anda third voltage application step 32.

The first immersion step 22 includes immersing the aluminum alloyworkpiece in a phosphoric acid deoxidizing solution. At the firstvoltage application step 24, a voltage is applied to the aluminum alloyworkpiece in the phosphoric acid deoxidizing solution. The phosphoricacid deoxidizing solution and the voltage act to remove surfacecontaminants and native oxide on the aluminum alloy workpiece. Inaddition, the phosphoric acid deoxidizing solution and the voltage actto form a thin porous oxide layer with very fine filaments on thealuminum alloy workpiece.

The second immersion step 26 includes immersing the aluminum alloyworkpiece from step 24 in a phosphoric acid anodizing solution. At thesecond voltage application step 28, a voltage is applied to the aluminumalloy workpiece in the phosphoric acid anodizing solution. Thephosphoric acid anodizing solution and the voltage act to form a porousoxide layer on the aluminum alloy workpiece. For example, the porousoxide layer has aluminum oxides and phosphates.

The aluminum alloy workpiece is then removed from the phosphoric acidanodizing solution and in the third immersion step 30 is immersed in acontrolled anodizing solution. At the third voltage application step 32,a voltage is applied to the aluminum alloy workpiece in the controlledanodizing solution. The controlled anodizing solution and the voltageact to form a dense oxide layer under the porous oxide layer.

The resulting coating is a duplex coating with the porous oxide layerexposed at the surface and the dense oxide layer formed underneath. Theporous oxide layer is relatively fragile and can be susceptible todissolution in during the anodization. In this regard, dissolution ofthe porous oxide layer during the formation of the dense oxide layer iscontrolled by using tartaric acid in the controlled acid solution. Thetartaric acid facilitates the formation of the dense oxide layer, butits action is not so severe such to dissolve the porous oxide layer.Therefore, the thickness of the porous oxide layer is substantiallyequivalent before and after the formation of the dense oxide layer.

As an example, the porous oxide layer has a filament structure of anamorphous oxide. The filament structure is also substantially preservedby use of the controlled acid solution. By preserving these features andthickness of the porous oxide layer, the properties of the porous oxidelayer can also be preserved in the resulting duplex coating. In thisregard, the duplex coating that is formed has an electric resistance ofat least 10⁹ ohms Particularly where the duplex layer is used both as acorrosion resistant layer and for adhesive bonding with another,dissimilar and electrically conductive material, the high electricalresistance of the duplex layer serves as a galvanic barrier between theunderlying aluminum alloy and the overlying electrically conductivematerial. Thus, the duplex layer in some examples can serve the multiplefunctions of corrosion resistance, adhesion promotion, and galvanicprotection.

In a further example, the controlled anodizing solution includes thetartaric acid and also sulfuric acid in a mixed acid solution. Forexample, the concentration of the tartaric acid in the mixed acidsolution can be 60-100 gram/L. In further examples, the controlledanodizing solution includes only the tartaric acid and the sulfuricacid, and possibly impurities. The ratio of tartaric acid to thesulfuric acid is from 1:1 to 4:1, and can be 2:1 for best control overpreserving the porous oxide layer. In further examples, the tanktemperature of the controlled anodizing solution during the formation ofthe dense oxide layer is 20-35° C.

The resulting duplex layer can be further treated to improve theproperties as desired. In one example, the duplex coating is furthertreated by immersion in a nitrilotrismethylene (NTMP) solution, as instep 34. The NTMP solution acts to stabilize the porous oxide layer, toenhance bonding with a later-applied adhesive, such as epoxy, and toimprove the corrosion barrier properties of the duplex oxide layer.Without being bound, the NTMP adsorbs onto the porous oxide layer toform a monolayer that renders the porous oxide layer hydrophobic andpromotes bonding with epoxy or other later-applied adhesives.

Alternatively, or in addition to the NTMP solution, the duplex coatingcan also be treated to further enhance corrosion resistance by immersionin an aqueous trivalent chromium-containing sealing solution. In thisregard, the aqueous chromium solution seals the dense oxide layerthrough formation of a chromium compound in the dense oxide layer.Therefore, the NTMP solution and the aqueous chromium solution can beused singly or in cooperation, with the NTMP solution enhancing bondingand the aqueous chromium solution enhancing corrosion resistance.

FIGS. 4 and 5 are micrographs of a workpiece at various points throughthe example method 20. FIG. 4 shows a workpiece having an aluminum alloysubstrate 42 and a porous oxide layer 44 formed during the voltageapplication step 28 but prior to the formation of a dense oxide layer46. FIG. 5 shows the workpiece after the formation of the dense oxidelayer 46. The thickness of the porous oxide layer 44, along a directionsubstantially perpendicular to the surface of the aluminum alloysubstrate 42, is substantially equivalent before and after the formationof the dense oxide layer 46.

The following examples illustrate further embodiments of the method 20.

An Al alloy sheet (Al2024) was washed with organic solvent to removesurface paints or stains. The sheet was then etched with sodiumhydroxide aqueous solution and rinsed with water. The etched Al alloysheet was then deoxidized in nitric acid solution and rinsed with water.The Al alloy sheet was then electrochemically deoxidized in phosphoricacid under the following conditions:

15 v % phosphoric acid aqueous solution;

29° C. solution temperature;

voltage ramp from 0V to 7.5V within a minute;

maintain voltage at 7.5V for 15 minutes.

The Al alloy sheet was removed from the deoxidizing bath and rinsed withwater.

The Al alloy sheet was then anodized in phosphoric acid anodizingsolution under the following condition, to form the porous oxide layer:

7.5 v % phosphoric acid aqueous solution;

room temperature (approximately 23° C.);

voltage ramp at approximately 5V/min to 15V within 3 minutes;

maintain the voltage at 15V for 20 minutes.

The Al alloy sheet was removed from the phosphoric acid anodizing bathand rinsed with water. The Al alloy sheet was then immersed in thecontrolled anodizing solution of a mixture of sulfuric acid and tartaricacid, under the following conditions:

tartaric acid 80 g/L+Sulfuric acid 40 g/L;

35° C. electrolyte bath temperature;

voltage ramp at approximately 5V/min to 13V within 3 minutes;

maintain the voltage at 13V for 20 minutes.

The Al alloy sheet was removed from the controlled anodizing solutionand rinsed with water.

The Al alloy sheet was then immersed in a 300 ppm nitrilotrismethylenephosphoric acid (NTMP) at room temperature for 15 minutes for sealing.

FIG. 2 illustrates an example anodized article 40 produced by the method20. In this example, the anodized article 40 includes the aluminum alloysubstrate 42 with a surface portion 42 a that is converted to the porousoxide layer 44, corresponding to the steps 22 to 28 above. The porousoxide layer 44 includes aluminum oxides/phosphates that are formedduring the voltage application step 28 of the method 20. The porousoxide layer 44 can be 0.2-0.8 micrometers in thickness and morespecifically may be 0.3-0.5 micrometers in thickness.

The anodized article 40 also includes the dense oxide layer 46 that isunder the surface portion 42 a. The dense oxide layer 46 can be 1-4micrometers in thickness, but is usually 2-3 micrometers for enhancedfatigue resistance. The dense oxide layer 46 can include residualtartaric acid and sulfate ions from the method 20 described above. Theporous oxide layer 44 and the dense oxide layer 46 together are a duplexcoating 48 that has an electric resistance of at least 10⁹ ohms

The article 40 also includes an electrically conductive material 50adjacent the duplex coating 48. For example, the electrically conductivematerial 50 is bonded to the duplex coating 48 with an intermediateadhesive layer 52. The intermediate adhesive layer 52 can be apolymeric-based adhesive. One example polymeric-based adhesive isepoxy-based adhesive, but this disclosure is not limited to epoxy-basedadhesives.

The electrically conductive material 50 is different in composition fromthe aluminum alloy of the substrate 42. Due to the electricalconductivity of the electrically conductive material 50 and of thealuminum alloy substrate 42, along with the close proximity of thesematerials to each other, a galvanic couple could form and acceleratecorrosion. However, the relatively high electric resistance of theduplex coating 48 provides a galvanic corrosion barrier between thealuminum alloy of substrate 42 and the electrically conductive material50 to prevent galvanic corrosion.

FIG. 3 illustrates a modified example of an anodized article 140. Inthis disclosure, like reference numerals designate like elements whereappropriate and reference numerals with the addition of one-hundred ormultiples thereof designate modified elements that are understood toincorporate the same features and benefits of the correspondingelements. In this example, the article 140 also includes a surfaceportion 142 a that has a porous oxide layer 144, but the porous oxidelayer 144 has been treated with the NTMP solution as described above, asrepresented at areas 144 a. The article 140 also includes a dense oxidelayer 146. Alternatively to the NTMP treatment, or in addition thereto,the dense oxide layer 146 can be treated with an aqueous chromiumsolution to locally form chromium compounds, represented at 146 a. Thechromium compounds seal the dense oxide layer 146 and further enhancethe corrosion resistance of the duplex coating 48.

FIG. 6 illustrates another example of an anodized article, namely ananodized airfoil 240. The anodized airfoil include an aluminum alloyairfoil (substrate) 242 that extends between a leading end 250 and atrailing end 252, with at least a surface portion of the leading end 250being converted to a porous oxide layer of aluminum oxides/phosphates,as described herein above. In this example, the anodized airfoil 240 issubstantially as described with reference to the article 40 of FIG. 2,and the electrically conductive material 50 is a sheath that is mountedadjacent the duplex coating at the leading end 250 of the anodizedairfoil 240. For example, the electrically conductive material 50 of thesheath is titanium or a titanium-based alloy. The duplex coating (FIG.2) provides a galvanic corrosion barrier between the aluminum alloyairfoil 242 and the electrically conductive material 50 of the sheath.

Although a combination of features is shown in the illustrated examples,not all of them need to be combined to realize the benefits of variousembodiments of this disclosure. In other words, a system designedaccording to an embodiment of this disclosure will not necessarilyinclude all of the features shown in any one of the Figures or all ofthe portions schematically shown in the Figures. Moreover, selectedfeatures of one example embodiment may be combined with selectedfeatures of other example embodiments.

The preceding description is exemplary rather than limiting in nature.Variations and modifications to the disclosed examples may becomeapparent to those skilled in the art that do not necessarily depart fromthe essence of this disclosure. The scope of legal protection given tothis disclosure can only be determined by studying the following claims.

What is claimed is:
 1. A method of anodizing comprising: immersing analuminum alloy workpiece in a phosphoric acid anodizing solution;applying a voltage to the aluminum alloy workpiece in the phosphoricacid anodizing solution, the phosphoric acid anodizing solution and thevoltage acting to form a porous oxide layer on the aluminum alloyworkpiece; removing the aluminum alloy workpiece from the phosphoricacid anodizing solution and then immersing the aluminum alloy workpiecein a controlled anodizing solution; applying a voltage to the aluminumalloy workpiece in the controlled anodizing solution, the controlledanodizing solution and the voltage acting to form a dense oxide layer onthe aluminum alloy workpiece under the porous oxide layer; andcontrolling dissolution of the porous oxide layer during the formationof the dense oxide layer by using tartaric acid in the controlled acidsolution such that the thickness of the porous oxide layer issubstantially equivalent before and after the formation of the denseoxide layer.
 2. The method as recited in claim 1, wherein the controlledanodizing solution includes the tartaric acid and sulfuric acid.
 3. Themethod as recited in claim 1, wherein the applying of the voltage to thealuminum alloy workpiece in the controlled anodizing solution includesramping the voltage to a predetermined hold voltage within threeminutes, and then holding at the predetermined hold voltage for no morethan 30 minutes.
 4. The method as recited in claim 1, wherein thecontrolled anodizing solution has a temperature of 20-35° C. during theapplying of the voltage.
 5. The method as recited as claim 1, whereinthe tartaric acid has a concentration in the controlled acid solution of60-100 gram/L.
 6. The method as recited in claim 1, wherein thecontrolled anodizing solution consists essentially of the tartaric acidand sulfuric acid.
 7. The method as recited in claim 6, wherein thecontrolled anodizing solution has a ratio of the tartaric acid to thesulfuric acid from 1:1 to 4:1.
 8. The method as recited in claim 6,wherein the controlled anodizing solution has a ratio of the tartaricacid to the sulfuric acid of approximately 2:1.
 9. The method as recitedin claim 1, wherein the phosphoric acid anodizing solution is a 7.5volume % phosphoric acid aqueous solution, and the phosphoric acidanodizing solution is at room temperature of 20-25° C. during theapplying of the voltage to the aluminum alloy workpiece in thephosphoric acid anodizing solution.
 10. The method as recited in claim1, wherein the phosphoric acid anodizing solution consists essentiallyof an aqueous phosphoric acid solution, and the controlled anodizingsolution consists essentially of the tartaric acid and sulfuric acid.11. The method as recited in claim 1, further comprising immersing thealuminum alloy workpiece that has the porous oxide layer and the denseoxide layer in a nitrilotrismethylene solution.
 12. The method asrecited in claim 1, further comprising immersing the aluminum alloyworkpiece that has the porous oxide layer and the dense oxide layer inan aqueous trivalent chromium-containing sealing solution to deposit achromium compound in the dense oxide layer.
 13. An anodized articlecomprising: an aluminum alloy substrate with a surface portion that isconverted to a porous oxide layer of aluminum oxides/phosphates; a denseoxide layer under the surface portion, wherein the porous oxide layer ofaluminum oxides/phosphates and the dense oxide layer together are aduplex coating that has an electric resistance of at least 10⁹ Ohms; andan electrically conductive material adjacent the duplex coating, theelectrically conductive material being different in composition from thealuminum alloy, and the electric resistance of the duplex coatingproviding a galvanic corrosion bather between the aluminum alloysubstrate and the electrically conductive material.
 14. The anodizedarticle as recited in claim 13, wherein the dense oxide layer includesresidual tartaric acid and sulfate ions.
 15. The anodized article asrecited in claim 13, wherein the dense oxide layer is sealed with achromium compound.
 16. The anodized article as recited in claim 13,wherein the dense oxide layer is thicker than the porous oxide layer.17. An anodized airfoil comprising: an aluminum alloy airfoil extendingbetween a leading end and a trailing end, with at least a surfaceportion of the leading end being converted to a porous oxide layer ofaluminum oxides/phosphates; a dense oxide layer under the surfaceportion, wherein the porous oxide layer of aluminum oxides/phosphatesand the dense oxide layer together are a duplex coating that has anelectric resistance of at least 10⁹ Ohms; and a sheath formed of anelectrically conductive material and mounted adjacent the duplex coatingat the leading end of the aluminum alloy airfoil, the electricallyconductive material being different in composition from the aluminumalloy, and the electric resistance of the duplex coating providing agalvanic corrosion barrier between the aluminum alloy airfoil and theelectrically conductive material of the sheath.
 18. The anodized airfoilas recited in claim 17, wherein the dense oxide layer includes residualtartaric acid and sulfate ions.
 19. The anodized airfoil as recited inclaim 17, wherein the dense oxide layer is sealed with a chromiumcompound.
 20. The anodized airfoil as recited in claim 17, wherein thedense oxide layer is thicker than the porous oxide layer.